Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
1998-06-24
2001-04-10
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S290000, C359S291000, C359S295000, C359S197100, C359S224200, C359S318000, C359S572000, C359S573000
Reexamination Certificate
active
06215579
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and apparatus for modulating a light beam for forming a two-dimensional image. More particularly, the invention relates to a columnar diffraction grating for performing such modulation.
BACKGROUND OF THE INVENTION
Many applications exist for devices which modulate a light beam, e.g. by altering the amplitude, frequency or phase of the light. An example of such a device is a reflective deformable grating light modulator
10
, as illustrated in FIG.
1
. This modulator
10
was proposed by Bloom et al., in U.S. Pat. No. 5,311,360. The modulator
10
includes a plurality of equally spaced apart, deformable reflective ribbons
18
which are suspended above a substrate
16
having reflective surface portions. An insulating layer
11
is deposited on the silicon substrate
16
. This is followed by the deposition of a sacrificial silicon dioxide film
12
and a low-stress silicon nitride film
14
. The nitride film
14
is patterned to form the ribbons and portions of the silicon dioxide layer
12
are etched such that the ribbons
18
are held by a nitride frame
20
on an oxide spacer layer
12
. For modulating light having a single wavelength &lgr;
0
, the modulator is designed such that the thickness of the ribbons
18
and the thickness of the oxide spacer
12
both equal &lgr;
0
/4.
The grating amplitude of this modulator
10
, defined as the perpendicular distance, d, between the reflective surfaces
22
on the ribbons
18
and the reflective surfaces of the substrate
16
, is controlled by applying voltage between the ribbons
18
(the reflective surface
22
of the ribbons
16
serves as a first electrode) and the substrate
16
(a conductive film
24
beneath the substrate
16
serves as a second electrode). In its undeformed state, with no voltage applied, the grating amplitude equals &lgr;
0
/2 and the total path length difference between light reflected from the ribbons and the substrate equals &lgr;
0
, resulting in these reflections adding in phase. Thus, in the undeformed state, the modulator
10
reflects light as a flat mirror. The undeformed state is illustrated in
FIG. 2
with incident and reflected light indicated as
26
.
When an appropriate voltage is applied between the ribbons
18
and the substrate
16
, an electrostatic force deforms the ribbons
18
into a down position in contact with the surface of the substrate
16
. In the down position, the grating amplitude is changed to equal &lgr;
0
/4. The total path length difference is one-half the wavelength, resulting in the reflections from the surface of the deformed ribbons
18
and the reflections from the substrate
16
interfering destructively. As a result of this interference the modulator diffracts the incident light
26
. The deformed state is illustrated in
FIG. 3
with the diffracted light in the +/−1 diffraction modes (D
+1
, D
−1
) indicated as
28
and
30
, respectively.
Adhesion between the ribbons
18
and the substrate
16
during wet processing utilized to create the space below the ribbons
18
and during operation of the modulator
10
has been found to be a problem in these devices. Numerous techniques to reduce adhesion have been proposed, including: freeze-drying, dry etching of a photoresist-acetone sacrificial layer, OTS monolayer treatments, use of stiffer ribbons by using shorter ribbons and/or tenser nitride films, roughening or corrugating one or both of the surfaces, forming inverted rails on the underneath of the ribbons, and changing the chemical nature of the surfaces. Sandejas et al. in “Surface Microfabrication of Deformable Grating Light Valves for High Resolution Displays” and Apte et al. in “Grating Light Valves for High Resolution Displays”, Solid State Sensors and Actuators Workshop, Hilton Head Island, S.C. (June 1994), have demonstrated that such adhesion may be prevented by reducing the area of contact by forming inverted rails on the underneath of the bridges and by using rough polysilicon films, respectively.
Furthermore, as Apte et al. recognize, a feature of the mechanical operation of the modulator
10
is hysteresis in the deformation of the ribbons
18
as a function of applied voltage. The theorized reason for the hysteresis is that the electrostatic attractive force between the ribbons
18
and the substrate
16
is a non-linear function of the amount of deformation, while the restoring force caused by stiffness and tension of the ribbons
18
is a substantially linear function.
FIG. 4
illustrates a simulated hysteresis characteristic where the light output (an indirect indicator of the amount of deformation of the ribbons
18
) is shown on the vertical axis and the voltage between the ribbons
18
and the substrate
16
is shown on the horizontal axis. Thus, when the ribbons
18
are deformed into the down position in contact with the substrate
16
, they latch in place, requiring a smaller holding voltage than the original applied voltage.
Bloom et al., in U.S. Pat. No. 5,311,360 teach that this latching feature is desirable as it gives the modulator
10
the advantages of active matrix design without the need for active components. In addition, Bloom et al. teach that this latching feature is also desirable in low power applications where efficient use of available power is very important. Recognizing the adhesion problem, however, Bloom et al., teach adding small ridges below the ribbons
18
to reduce the contact area and thereby reduce the adhesion problem. Because the substrate of the modulator
10
is used as an optical surface, however, the manufacturing processes for adding small ridges to the surface is complicated by the requirements that the reflecting portions of the substrate
16
be smooth with high reflectivity and be in a plane parallel to the ribbons
18
.
Conventional displays are formed in two dimensional arrays of pixels. The discrete image formed by each of the myriad of pixels are integrated by the eye of the user to form a composite of the pixels representing an overall image. Unfortunately, the cost of such a display system increases because as each pixel is replicated to form the entire array the cost of making each pixel is similarly replicated. Examples of such pixellated displays are televisions and computer monitors. The pixels for each can be formed of LCD devices, or by a CRT.
Therefore, what is needed is a diffraction grating light valve in which adhesion between reflective elements and a substrate is reduced or eliminated without resorting to complicated surface treatments required for reducing such adhesion.
What is also needed is a display that lowers the cost of manufacture by reducing the number of pixels required to build the system without lowering the image quality.
SUMMARY OF THE INVENTION
The invention is a diffraction grating light valve (GLV) and method of use thereof for modulating an incident light beam for forming a two-dimensional image. The diffraction grating light valve includes a plurality of elongated elements each of which have a reflective surface. The elongated elements are suspended substantially parallel to each other above a substrate with their respective ends supported and substantially aligned so as to form a column of adjacent reflecting surfaces (GLV array). The elongated elements are grouped according to display elements. Alternate ones of each group are deformable by a applying a voltage with respect to the substrate. An approximately flat center portion of each deformed elongated element is substantially parallel to and a predetermined distance from a center portion of each undeformed element. The predetermined distance is selected to be approximately one-third to one-fourth of the distance between the undeformed reflective surfaces and the substrate such that deformed elongated elements do not contact the surface of the substrate. Avoiding contact with the substrate prevents the elongated elements from adhering to the substrate. In addition, limiting the predetermined distance avoids hysteresis in deformin
Bloom David M.
Godil Asif
Epps Georgia
Haverstock & Owens LLP
Lester Evelyn A.
Silicon Light Machines
LandOfFree
Method and apparatus for modulating an incident light beam... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for modulating an incident light beam..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for modulating an incident light beam... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2548523